1. Field of the Invention
The invention relates to a control system for a variable pitch axial fan used with the air flow system of a utility boiler.
2. Description of the Prior Art
Fans for furnishing combustion air to utility boilers and for aiding in the removal of combustion gases are typically sized with a margin of safety which will permit continued operation under adverse conditions which may occur and which may require a greater draft than normally required for the boiler. Thus, so-called test block specfications are established which result, in effect, in the selected fans being somewhat oversized in capacity, such as 15 to 20% for example, with respect to normally expected operating conditions. Also, during swing load operating periods the boiler may be operating well below its rated capacity.
Thus, in making a selection between an inlet vane controlled centrifugal fan and a variable pitch axial flow fan, one factor which is given consideration is the static efficiency of the fans under various operating conditions below rated capacity as well as at rated capacity. While variable pitch axial flow fans may have a lower static efficiency than the centrifugal fan at test block conditions, at a condition of normal 100% boiler load the efficiency advantage is reversed. Further, as the boiler load drops further and further below 100%, the efficiency advantage of the axial fan increases more and more. For this reason, as well as others, the variable pitch axial flow fan is considered to be more desirable than the centrifual fan in some application.
However, axial flow fans are more subject to the stall phenomenon resulting from a condition in which the blade attack angle is too steep relative to the mass air flow (and hence velocity). Under the stall condition a static pressure rise across the fan is excessive for the flow rate and potential damage from the vibrations associated with a continued stall condition can occur. Therefore it is apparent that prevention of a stall condition of variable pitch axial fans in the utility boiler draft applications is desirable.
This well known problem of stall in connection with axial flow fans has been dealt with in several ways. One known way of preventing stall in connection with an axial flow turbo compressor having adjustable inlet guide vanes and adjustable stator vanes and used in blast furnace service is to use a blow-off valve to atmosphere between the compressor and the blast furnace to reduce the discharge pressure when it is excessive relative to the flow. However, it is considered undesirable in a utility boiler application to attempt to blow off to atmosphere since on the forced draft side of the boiler furnace this will create other repercussions, as in fuel-air ratios for example, and on the induced draft side of a boiler furnace it would be combustion gases being blown off before their treatment in the passage to the stack.
In an arrangement for an air conditioning and ventilating system for a building as disclosed in U.S. Pat. No. Re. 28,946, provision is made for sensing flow rate and static duct pressure in the duct downstream from a fan, which may be a variable pitch axial fan. The arrangement is intended to maintain a flow rate that is correlated with the static pressure to maintain efficient operation without a surging condition. In the system, the temperature changes in the spaces being served control inlet dampers to the served spaces. Accordingly, the system resistance changes in accordance with temperature demands. As the system resistance is increased due to dampers closing, the flow will be reduced of course while the discharge static pressure increases. This results in the pitch of the fan blades being changed to give a discharge static pressure that is less than at the first reduced flow, which of course results in a further reduced flow. If that is satisfactory for supplying the conditioning, then the system will continue at that same condition. However, if that further reduced air flow is inadequate for conditioning purposes, then the temperature change requirements result in a reduced system resistance and the static pressure will drop and the flow will increase. Thus, with that control arrangement the temperature changes control the system resistance to which the flow responds automatically and in a direction to satisfy the temperature control needs. In the system the duct pressure is continually being used for readjusting the position of louvers or the pitch of the axial fan blades.
In the system according to the present invention, the blade pitch changes will follow the demand of air for the given boiler load and the static pressure rise changes across the fan with the different air flow changes is of no consequence and does not influence the control unless and until a stall condition is approached. Then and only then will the relation of static pressure rise across the fan to air flow volume become a factor in the control and the normal boiler load control of the blade pitch be overridden and blocked by the stall prevention control.